It is known that stationary gas turbines are used to generate mechanical energy, which a generator generally converts into electrical energy. For this purpose, a fossil fuel is burnt in the gas turbine with an air stream that has been compressed by the compressor, to form a hot gas which then expands so as to perform work at the rotor in a turbine. The gas turbine is operated in such a way that sufficient energy is output at the rotor shaft to generate the electrical energy, while a maximum temperature of the hot gas at the turbine inlet should not be exceeded.
The turbine inlet temperature cannot be measured directly, on account of its high values. Therefore, the temperature of the exhaust gas which is present at the turbine outlet is recorded, and the turbine inlet temperature can then be determined by calculation from this turbine outlet temperature. The turbine outlet temperature and therefore indirectly also the turbine inlet temperature can be controlled by means of the quantity of fuel introduced into the combustion chamber, while these temperatures are also dependent on the temperature of the air at the compressor inlet. To simplify control of the gas turbine, an auxiliary variable is calculated by means of a mathematical model, eliminating the dependency of the turbine outlet temperature on the compressor inlet temperature.
This auxiliary variable is used as a corrected turbine outlet temperature. It is dependent only on the quantity of fuel consumed, resulting in simple control of the gas turbine. Although the control is also dependent on the mains frequency of the power generated by the generator, this influence is not taken into account here.
To increase the performance of the gas turbine, it is also possible for water to be fed to the air stream sucked in by the compressor even before it has been compressed, in order to increase the mass flow through the gas turbine. This is generally known as wet compression mode.
The temperature of the air which is sucked in generally differs from the temperature of the liquid which is injected. Since the temperature-measuring devices arranged at the inlet of the compressor for measuring the air temperatures are wetted by the liquid that has been introduced, the temperature-measuring devices are not recording the temperature of the air, but rather that of the liquid.
If, on account of a compressor inlet temperature which is clearly higher following the measurement, a lower turbine outlet temperature is determined than the turbine outlet temperature which is actually present, the controller of the gas turbine increases the supply of fuel into the combustion chamber in order to compensate for the presumed difference. However, this leads to overfiring of the gas turbine, i.e. the actual turbine inlet temperature may become higher than the maximum permissible turbine inlet temperature. The gas turbine is underfired if the compressor inlet temperature measured is lower than the actual turbine inlet temperature.
Overfiring of the gas turbine can lead to overheating of the components exposed to hot gas and therefore to a reduction in their service life, or even to defects. On the other hand underfiring of the gas turbine leads to loss of performance.